This invention relates to an air conditioner the capacity of which can be controlled to be optimized with respect to a wide range of changes in the load.
Conventionally, methods for controlling air conditioners according to changes in the load were developed, including one adopted for air conditioners having a multi-cylinder reciprocating compressor and based on changing the number of effective cylinders (unloading), and one based on changing the number of poles of the motor by changing over connections thereof from the outside to select the operating speed of the compressor.
Thereafter, with development of rotating type compressors such as rotary compressors and scroll compressors, inverter driving systems for selecting the rotational speed of the compressor by changing the frequency of the current for power supply to the compressor driving motor have been put to practical use. A type of air conditioner, such as the one disclosed in Japanese patent Unexamined Publication No. 63-140884, having a scroll compressor with an inverter driving system has been proposed prior to the present invention. Specifically, in FIG. 6 of this publication, a refrigerant circuit of an air conditioner having an inverter driving system is illustrated which is composed of a scroll compressor, an indoor heat exchanger, an expansion valve, and an outdoor heat exchanger. A refrigerant circulation passage is reversed for cooling or heating operation by a four-way change-over valve provided in the refrigerant circuit. The speed at which the scroll compressor is rotated can be changed by an inverter interposed between an ac power source and a motor for driving the compressor. The frequency of the driving current output from the inverter is controlled according to the indoor cooling load or heating load. Ordinarily, when the compressor rotates at a low speed, it operates at a low pressure ratio, that is, the discharging pressure is low while the drawing pressure is high or, when the compressor rotates at a high speed, it operates at a high pressure ratio, that is, the discharging pressure is high while the drawing pressure is low.
To change the capacity of the air conditioner in response to a change in the load, the discharge rate of the compressor may be changed. However, in a case where a reciprocating compressor is controlled by the method of changing the rotational speed of the compressor with the inverter, the range of changes in the rotational speed is restricted due to vibrations of a reciprocating member and noise generated therefrom. In the case of a rotating type compressor such as a rotary or scroll compressor, the range of changes in the rotational speed is comparatively extended. With development of multiple room air conditioners for air-conditioning a plurality of rooms, however, there is a need for a further improvement in the capacity variation range.
To meet this requirement, the range of changes in the rotational speed of the compressor may be extended by extending the range of frequencies with which the driving current is supplied from the inverter to the compressor. During high speed operation, however, problems relating to the strengths of mechanical parts and to the reliability of the bearing are encountered because the load increases as centrifugal forces applied to movable parts increase, and the problem of increases in mechanical and fluid losses are also encountered resulting in a reduction in the efficiency such that a larger input is required for the same work. During low speed operation, the following problems are encountered. That is, the rate at which the refrigerant gas leaks out of the compression space is increased, resulting in a reduction in the efficiency. It is difficult to form the desired oil film on the sliding portion and, hence, to achieve the desire reliability. In the case of a compressor with a lubrication oil pump utilizing a centrifugal force, it is difficult to maintain the desired oil supply rate.
Thus, it is undesirable to design the compressor to extend the capacity variation range only by extending the range of changes in the rotational speed of the compressor.